This disclosure relates generally to methods and apparatus for acquiring cores from subterranean formations. More particularly, this disclosure relates to methods and apparatus for mitigating the effects of core jamming by utilizing a frangible core barrel.
Formation coring is a well-known process for obtaining a sample of a subterranean formation for analysis. In coring operations, a specialized drilling assembly is used to obtain a cylindrical sample of material, or “core,” from the formation so that the core can be brought to the surface. Once at the surface, the core can be analyzed to reveal formation data such as permeability, porosity, and other formation properties that provide information as to the type of formation being drilled and/or the types of fluids contained within the formation. Coring operations include bottom-hole coring, where a sample is taken from the bottom of the wellbore, and sidewall coring, where a sample is taken from the wall of the wellbore. Coring operations can also be performed using conventional wellbore tubulars, such as drill string, or using wireline conveyed tools.
In bottom-hole coring, as a core is being cut, it is received within an elongated tubular receptacle, known as a barrel. As the core moves into the barrel it can become stuck, or “jammed,” in the barrel, and prevent additional core from moving into the barrel. Once a jam occurs, the cut core is subjected to increased compressive loads until the coring operation is stopped. Often, the increased compressive loads can damage the core before the coring operation can be stopped. Thus, in many instances, a core jam can result in an insufficient length of core being obtained and/or damage the core that can compromise the desired analysis. Therefore, in bottom-hole coring operations, when a core jam is detected, the coring operation is halted and the tools are brought back to the surface. This can be especially costly in deep wells where it may take several hours to retrieve the coring tools from the bottom of the well.
Thus, there is a continuing need in the art for methods and apparatus for acquiring cores that overcome these and other limitations of the prior art.